Domains of seismic noise response in faulted limestone (central Apennines, Italy): insights into fault-related site effects and seismic hazard

Studying seismic response at complex geological settings is a challenge due the occurrence of site effects related to widespread faulting/fracturing characteristics of the rock masses. Fault-related site effect is always a crucial aspect of an assessment of seismic hazard, but especially in assessments of areas where urban settlements are located in the proximity of regional fault zones. In order to detail the correlation between fault properties and seismic noise response (in terms of directional amplification), we have used a multidisciplinary approach to study a pervasively faulted limestone sequence cropping out in the central Apennines (Italy). We integrated results from (1) geological and structural surveys, (2) in situ geomechanical analyses and (3) geophysical measurements (ambient noise measurements processed with the H/V technique) performed along and across a 50 m-thick, NW–SE-striking fault zone cutting through a limestone sequence. We then reconstructed the architecture of the fault zone by individualising different structural domains (a fault core and two damage zones) and, eventually, we evaluated the fracture intensity across the fault zone by correlating structural (discontinuity spacing, discontinuity pervasivity, size of lithons) and geomechanical (rebound hardness index provided by the Schmidt hammer) parameters. Ambient noise measurements documented a variability of directional amplification across the fault zone and in the surrounding undeformed rock mass, making it possible to recognise possible site effects. The results show the occurrence of a main NE–SW-trending directional amplification oriented perpendicular to the strike of dominant slip structures within the fault core, whereas minor polarisation trends are transversal-to-perpendicular to the strike of subsidiary structures within the damage zones. The results support evidence of structurally-controlled directional amplification due to the stiffness anisotropy produced by the orientation of fault-related structures. When compared with results from published studies, our dataset can be used for understanding factors (i.e. the meso-scale fault properties) leading to directional amplification within a fault zone. Accordingly, we discuss our results in terms of the seismic response of the fault zones and the mitigation of seismic hazard in areas associated to tectonic activity.